Project Summary The dimorphic fungus, Histoplasma capsulatum (Hc) is endemic to the Midwestern and southeastern United States. Most infections are mild or asymptomatic. The organism establishes a latent state and can cause a life-threatening infection in immunocompetent or immunosuppressed individuals. We have shown that the chemokine receptor, CCR2, is critically important in controlling pulmonary infection with Hc in mice. The absence of this receptor is associated with exacerbation of infection and the induction of interleukin (IL)-4 by unconventional sources-macrophages and dendritic cells. Neutralization of IL-4 restores immunity. Moreover, we have evidence that signaling by the chemokines CCL12 and 2, two CCR2 ligands, are important in optimal host defenses. In the following proposal, we will explore the mechanisms by which CCR2 signaling is important in constraining IL-4 generation and promoting an effective cellular immune response. Specific aim 1 will examine the role of CCL2 and 12 in host defenses and inflammation. We will identify the cells producing these chemokines in situ and ex vivo. We will investigate if the absence of the receptor or the chemokines alters the in vivo residence of yeast cells. In specific aim 2, we will utilize a mouse that has green fluorescent protein linked to the IL-4 gene to investigate the evolution and temporal appearance of IL-4- transcribing cells. Studies will be performed to determine if Hc modulates expression of CCR2. We also will investigate the pathways that macrophages and dendritic cells utilize to generate IL-4. In specific aim 3, we will examine mechanisms by which the absence of CCR2 and/or IL-4 contributes to defective immunity. We will assess the possibility that the absence of CCR2 promotes the emergence of alternatively activated macrophages or myeloid suppressor cells. We will determine if either or both of these populations dampen antifungal immunity. We will test T cell function and expansion. We will assess the possibility that the failure of T cells to expand in the lungs is consequence of active inhibition as well as modulation of surface receptors. The goal of these studies is to better understand the pathogenesis of Hc, thereby enhancing knowledge of how Hc escapes host defenses. These findings contribute to the influence of the chemokine- cytokine network in microbial pathogenesis and evince a new role for CCL12.